1. Field of the Invention
The present invention relates to a laser output control method for controlling a laser apparatus such as an excimer laser apparatus which generates a laser pulse while performing charging for each pulse, and to a laser apparatus and an exposure apparatus which uses the laser apparatus as a pulse light source.
2. Description of the Related Art
Heretofore, at the time of producing devices such as semiconductor devices, liquid crystal display devices, image sensing devices (CCD etc.), or thin-film magnetic heads using a photolithography technique, a projection exposure apparatus has been used which projects an image of a pattern on a reticle constituting a mask, which is imaged via a projection optical system, onto a wafer (or glass substrate etc.) which has been coated with a photoresist or the like, to thereby expose the wafer.
In general, a pulse oscillation type laser apparatus, such as a KrF excimer laser apparatus or an ArF excimer laser apparatus, is used as a light source for the exposure apparatus, and is provided with a high voltage charging control unit having a charging circuit. In these pulse oscillation type laser apparatus, emission of a laser pulse is performed by charging a high voltage to the charging circuit for each pulse and then switching at a desired timing to discharge across two electrodes installed inside a gas chamber.
With such laser apparatus, in the case where charging is performed for each pulse, the following methods are adopted:
(1) a method where a target charging voltage value is given from outside of the laser apparatus; or
(2) a method where a target energy value is given from outside the laser apparatus, and a target charging voltage value for the next pulse is computed inside the laser apparatus by executing computational processing, based on the history of the energy of laser pulses emitted from the laser apparatus, the gas condition inside the laser apparatus, and the value of the given target energy value.
Moreover, the charging operation for the next laser pulse is commenced at the following stages:
(1) the stage where the setting of the target charging voltage value from outside the laser apparatus is completed, or
(2) the stage where the computation of the target charging voltage value inside the laser apparatus is completed.
Recently, with semiconductor exposure apparatus, in order to improve the number of wafers processed per unit time (throughput), a pulse oscillation type laser apparatus with a higher repetition frequency is required for the light source. On the other hand, it is also required to improve the accuracy and stability of the principal characteristics of each laser pulse such as the energy, wave length and half band width, so that with this there is an increasing trend for control of the laser interior to become more complicated, and for the processing time to increase. Hence, shortening of the time for charging ranks much greater importance.
However in the case where as mentioned above, emission of a laser pulse is performed at a high repetition frequency, the following problem remains. That is to say charging may be carried out at:
(1) a stage where the setting of the target charging voltage value from outside the laser apparatus is not completed, or
(2) a stage where the computation of the target charging voltage value inside the laser apparatus is not completed, due to charging not being able to be commenced even though the timing for where charging must be started for the next pulse has eventuated, and the time related to charging being short. If this occurs, there is an undesirable situation such as the next laser pulse being emitted with there not being the required charging voltage value.
As a means of dealing with this problem the following has been considered:
(1) As a method of speeding up of the charging time itself, develop an improved charging apparatus, and shorten the charging time by modifying the hardware (laser apparatus).
(2) As a method of shortening the computation time for the target charging voltage value, and speeding up the setting timing for the energy directed from outside of the laser apparatus or for the target charging voltage value,
a) shorten the time for computing the charging voltage inside the laser apparatus,
b) shorten the time for computing the target charging voltage in the device outside the laser apparatus (speed up computation, simplify algorithms, increase interface speed etc.).
However, in the case where these means are used, the need arises to modify the equipment structure of the hardware such as the charging device and the computation circuit, with the problem of an increase in cost
The object of the present invention is to provide a laser output control method, a laser apparatus, and an exposure apparatus, each of which can shorten the time taken for charging for laser oscillation without modifying the equipment structure of hardware or the like.
Another object of the present invention is to provide an exposure apparatus which can improve the evenness of a cumulative exposure amount for a substrate such as a wafer.
The method of the present invention is for controlling the output of a laser pulse where laser pulses are continuously emitted while performing charging for each pulse, and this method comprises: a pre-charging step for commencing charging for a next pulse with a previously set first charging voltage value as a target; a voltage computation step for computing a second charging voltage value required for the next pulse during said pre-charging step; and a fine charging step for continuing said charging for the next pulse with said second charging voltage value as the target instead of said first charging voltage value, at the point when said second charging voltage value is computed in said voltage computation step.
On the other hand, the laser apparatus of the present invention is a laser apparatus which emits a laser pulse while performing charging for each pulse, and the apparatus comprises: a detector for detecting energy for each pulse; and a charging control system having a computer for computing a second charging voltage value required for a next pulse, which commences charging for said next pulse with a previously set first charging voltage value as the target, and during charging based on said first charging voltage value, computes using said computer a second charging voltage value required for said next pulse, based on the energy detected by said detector, and continues charging for said next pulse with said computed second charging voltage value as the target instead of said first charging voltage value, at the point when said second charging voltage value is computed.
Moreover, the present invention is an exposure apparatus provided with the abovementioned laser apparatus as a light source.
With the above laser output control method and laser apparatus, charging for the next pulse is commenced with a previously set first charging voltage value as the target, and during charging up until the first charging voltage value, the second charging voltage value required for the next pulse is computed. Furthermore, at the point when the second charging voltage value is computed, charging is continued with the second charging voltage value as the target instead of the first charging voltage value. Therefore charging can be commenced for the next pulse immediately after completion of the previous pulse emission. Moreover, since the target is switched from the first charging voltage value to the final second charging voltage value in a condition with a certain amount of charging, the time until completion of charging is considerably shortened.
Furthermore, since this is completed by only modifying the charging algorithm, without the necessity for modifying the equipment structure of the hardware etc., this can be realized at a low cost.
The exposure apparatus of the present invention is one where a mask is illuminated by pulse beams of a pulse light source and the pattern formed on the mask is transferred to a substrate, and incorporates the abovementioned laser apparatus as the pulse light source.
With this exposure apparatus, since the abovementioned laser apparatus is incorporated as the pulse light source, the pulse period can be shortened, and due to the resultant high light emission repetition frequency, an improvement in throughput can be realized . Moreover, the evenness of the cumulative exposure amount for the substrate can be improved.